1. Field of the Invention
The present invention relates to a power supply system and an image display device, more particularly, to a power supply system for driving a lamp unit without occurrence of over-current and a liquid crystal display (LCD) device employing the power supply system.
2. Description of the Related Art
In general, LCD devices include a liquid crystal module having a liquid crystal panel, data drivers, gate drivers, and a backlight unit. Since liquid crystal in an LCD device does not emit light by itself, a light generating component such as a backlight unit is necessitated in the LCD devices to display images. The backlight unit typically has at least one lamp generating light.
The LCD devices also include an inverter for driving lamps in the backlight unit. In an LCD device for a TV receiver, the current consumption for backlighting the LCD device amounts to several tens to several hundreds watts. In this case, instantaneous over-current (or “inrush current”) flows inner circuits when a power supply voltage signal is applied to a power supply circuit of the LCD device. To prevent the inrush current, slow start circuit is generally used in the conventional LCD devices.
FIG. 16 is a graphical view of signal waveforms of an inrush current signal and a power supply voltage signal in a conventional power supply system for LCD devices. In the conventional power supply system for LCD devices, there have been such drawbacks that an inverter of the power supply system is provided with a turn-on signal even when the power supply voltage signal is below a predetermined voltage level enough to appropriately operate the inverter. In this case, if the inverter is turned on when the power supply voltage signal has a value below the predetermined voltage level, considerable amount of current is required to make constant current flow lamps in an LCD device. This results in occurrence of the inrush current that often causes a fuse open-circuited.
The power supply voltage signal may also be lowered or interrupted momentarily besides the initial stage of applying the power supply voltage signal to the inverter. For example, when a failure of power supply occurs for a time period such as several milliseconds, an inverter controller allows inrush current to be supplied to make constant current flow the lamps. As a result, the inverter controller is often shut down and also remains at the shutdown state even when the power supply voltage signal is recovered to a normal level.
In a direct-type backlight system in which multiple lamps are driven in parallel using one inverter, the conventional power supply system has problems such that all the lamps connected to the inverter are turned off even when only one of the lamps fails.
FIG. 17 is a graphical view of signal waveforms of a lamp-on signal and a power supply voltage signal in a conventional power supply system for LCD devices. As shown in FIG. 17, when the lamp-on signal is applied to a lamp, the lamp is turned on in several hundreds of milliseconds. Thus, a transient time period is necessitated before the lamp becomes a normal on-state. During the transient time period, the lamp is recognized as being in off-state. As a result, all the lamps are erroneously turned off in the transient time period. Also, when pulse width modulation (PWM) dimming is performed for lamp luminance control, a lamp-on detection signal corresponding to a PWM dimming signal may cause an erroneous recognition of the lamp in a low-level period.
Therefore, a need exists for a power supply system which provides prevention of such an erroneous recognition of lamp-on/off state and erroneous shutting down of the inverter controller. Further, it will be advantageous to provide an LCD device having such a power supply system.
This application is based on and claims the priorities from Korean Patent Application Nos. 10-2003-0072695 and 10-2003-0089789 filed on Oct. 17, 2003 and Dec. 10, 2003, respectively, the disclosures of which are incorporated herein in their entireties by reference.